Ultrasonic monitoring techniques and apparatus for determining the composition of fluids are known. For example, U. S. Pat. No. 5,060,507 describes an apparatus that monitors fluid mixtures by comparing acoustic travel-times of both changing and reference fluid samples. U.S. Pat. No. 4,852,396 uses sonic velocity measurements made through windows in the side of a reactor to determine the fractional volumes of the liquid components of a two-liquid mixture in the reactor. Measured data for each of the two components are known. U. S. Pat. No. 4,522,068 describes use of ultrasonic transducers in a vessel to determine the density of a liquid slurry within the vessel. A method of measuring the composition of an oil and water mixture in a flowing pipeline is described in U.S. Pat. No. 4,656,869. The speeds of sound transmission in the pipeline and in the separate liquids are used to determine the water/oil ratio.
Previously, monitoring of fluid composition using ultrasonics has been attempted using transducers mounted directly in the fluid flow or through special windows machined into the wall of a conduit or vessel. These approaches are all subject to fouling of the sensors or windows by the fluid, making these systems ineffective for long-term, maintenance-free operation. Similarly, the intrusion of the transducers can result in contamination of the process being measured or disruption of the flow pattern in the conduit.
Systems described in the prior art are calibrated off-line by filling the space between transducers with reference fluids of known composition, measuring the transit time or sound speed of the known fluids, and then establishing a relationship between these measurements and the known fluid composition. The derived relationship is later used to convert the measurements into the composition of a mixture. The calibration is difficult due to the need to separate individual fluid components or prepare fluid samples of known composition. In addition, to obtain a suitable calibration relationship, the sample measurements must be obtained over a range of fluid pressure and temperature conditions that covers the range of pressures and temperatures of the process being monitored. In practice, the need to contain and measure the characteristics of fluid samples under process conditions can make the calibration expensive and difficult.
Those skilled in the art readily understand the wide range of potential applications for composition monitoring of fluids flowing in a conduit in various industries such as food, pharmaceuticals, chemicals, petroleum, waste treatment, and paper. Fluid composition is an important indicator of the quality of many industrial processes. An important application of ultrasonic monitoring in is the determination of the quality of a process separating oil and water in the petroleum industry. In this application, it is desirable to monitor low levels of water left in a fluid mixture stream after various separations and prior to the combination, transit, and sale of the fluid as crude oil. By monitoring the water content of the mixture at various stages and locations of the separation process, the separation process can be optimized and excess water content minimized.